The but instead of giving her the

The short story is about a group of girls, but it's mainly about two of the girls; Celia and Elizabeth.
The girls are about nine years old, but Celia has failed once, and is therefore one class behind the others.
Celia is always following the other girls, on their way home from school.
They hate it and don't want to be with her, because she is a nerd.
She is a diabetic, chubby and wears big glasses and that really makes her an easy victim.
Elizabeth is a part of the group, but if it wasn't for Celia, she knows that she would be the next in line to get mobbed and teased by the others. That really worries her.
Elizabeth's mother always tells Elizabeth to be nice to Celia and to treat her with respect, but Elizabeth is to stubborn to see that her mother is right.
One day at school a rich girl from Grade 4, who name is Sandra has some
milk-chocolate, which she is giving little pieces of to Elizabeth's classmates. Elizabeth asks if she can have some, but Sandra says that the last bit is for Celia. Celia explains that she can't eat it because she is a diabetic. Then Elizabeth again asks if she can get it, but instead of giving her the piece of chocolate, Sandra starts to say that Suckybabies can't have candy. The reason that Sandra is calling Elizabeth for a Suckybaby is because she sucked her thumb, when she was younger.
Later that day on the way home from school, the girls slide down a snowy slope.
Suddenly the other girls (except Celia)are beginning to mob Elizabeth by jelling "Sucky". Elizabeth looses her temper and starts to beat up Celia.
After some time Celia forgives Elizabeth, but Elizabeth will never forgive Celia for making her life miserable.
Celia dies at seventeen probably of diabetes.
The themes of "Celia Behind Me" has to be;